A plurality of pivotably connected slats can be movable between a closed position wherein the slats cover the vehicle box, and an open position. At least one slat can include a first and second end fittings having first and second fixed locking elements, respectively. Each of first and second side rails can include a movable locking element movable between a locked position in which the movable locking elements in the closed position engage an adjacent one of the fixed locking elements to restrict movement of the at least one slat away from each side rail, and an unlocked position in which the movable locking element is disengaged from the adjacent one of the fixed locking elements allowing movement of the at least one slat toward the open position.